The automatic call back feature allows a calling party to actuate a button on the calling telephone terminal when a busy signal is received back from the called telephone terminal. When the called telephone terminal finishes the present call, a call is then set up between the calling telephone terminal and the called telephone terminal. In the prior art, the automatic call back feature has long been provided by customer telecommunications switching systems (often referred to as PBX). However this feature is very complex when provided by a network of customer telecommunication switching systems, since it requires each of the customer telecommunication switching systems to be specially programmed for this feature. Because of this complexity, this feature is not provided across the public telephone network. U.S. Pat. No. 5,012,466 discloses the provision of automatic call back services utilizing a network of telecommunication switching systems. Another such system is set forth in U.S. Pat. No. 4,899,374.
Another method for providing automatic call back is illustrated in U.S. Pat. No. 4,899,374. This discloses the use of a separate control system with a telephone to provide the automatic call back feature. If the user is unable to complete the call when originally dialed, the control system repeatedly tries at predefined intervals to complete the call. The problem with that method is that the control system is repeatedly placing calls thus tying up the telecommunication switching system and further the predefined interval has to be reasonably long to avoid the problem of tying up the telecommunication system. The called party may hang up from one telephone call and start a second telephone call before the control system redials to attempt to do the automatic call back.
The problem of providing automatic call back service becomes even more complex when trying to perform this service for multi-media calls. The method proposed in U.S. Pat. No. 4,899,374 would constantly be utilizing large amounts of bandwidth each time a call was attempted. When the control system attempts to place the call, it needs to request the necessary bandwidth to support a multi-media call. A similar problem exists in U.S. Pat. Nos. 4,899,374 and 5,12,466. In addition, the facilities and bandwidth required for the call when it is finally completed by automatic call back may have changed from when the original call was made. Finally, if the call was made through a combination of private and public networks, providing the call back can be very difficult.
What is needed is the ability to be able to determine when both telephone terminals are idle so that the automatic call back service can be performed without tying up substantial networks facilities in performing the automatic call back service.